Presently, as secondary batteries of high-energy density, lithium-ion secondary batteries are widely employed that are made using a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent, so as to carry out charge-discharge by causing lithium ions to migrate between a positive electrode and a negative electrode. Specifically, lithium secondary batteries have already been practically used such as small power supplies of cell phones or notebook computers. Further, since the lithium secondary batteries can be used as power supplies such as power supplies for vehicles including electric vehicles and hybrid vehicles, or larger power supplies of distributed power storage, there have been increasing demands for the lithium secondary batteries.
However, the lithium secondary battery includes a number of rare metal elements such as lithium in its electrode-forming materials. Therefore, there is a concern that the supply instability of the materials according to increasing demands for large power supplies occurs.
A study on a sodium secondary battery has been promoted to solve the supply problem. A positive active material used for the sodium secondary battery contains sodium which is a rich and inexpensive resource, instead of lithium. Therefore, when the sodium secondary battery can be practically used, the large power supplies can be supplied in a large quantity.
Patent Documents 1 to 3 disclose that a composite metal oxide containing Na, Mn, and Fe has been used as a positive active material for the sodium secondary battery.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2009-135092
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-209037
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2010-080424